1. Field of the Invention
The present invention relates to extraction of selected hydrocarbons from mixed hydrocarbon and raffinate streams using carbon. More particularly, the present invention is directed to adsorbing polymeric hydrocarbons from hydrocarbon or raffinate streams using activated carbon as the adsorbent. Specifically, the present invention is directed to the extraction of poly-isobutylene from C.sub.4 and/or C.sub.5 raffinate using activated carbon as the adsorbent.
2. Discussion of Background and Material Information
Steam cracked naptha produces several product streams including a C.sub.4 raffinate stream. This C.sub.4 stream is a mixture of four carbon hydrocarbons and can be used in the production of methyl tert-butyl ether (MTBE) or poly-isobutylene (PIB). In MTBE production, the process steps include adding methanol to the C.sub.4 raffinate stream to produce MTBE and subsequent recovery of methanol. In the PIB process, the C.sub.4 stream is contacted to an acidic catalyst to convert isobutylene to poly-isobutylene. At steps during the PIB process a second C.sub.4 raffinate stream is recovered. This C.sub.4 stream typically contains butene-1, butene-2, isobutylene, n-butane, isobutane, and a low level of impurities including a small amount of PIB.
A C.sub.4 stream with a small PIB content, however, is considered unacceptable for MTBE production due in part to the high boiling point of PIB. Related to this, even small quantities, in parts per million, have been observed to plug a methanol recovery column in a processing plant and cause a gram/mole to more than 2,500 gram/mole with a typical weight of 900 gram/mole.
Distillation separation of PIB from the second C.sub.4 stream is impractical because of the relatively low levels of PIB present, the relatively high molecular weight of PIB, and because PIB is relatively visous and has a high boiling point relative to the C.sub.4 isomers.
U.S. Pat. No. 3,725,377, Cottle, assigned to Phillips Petroleum Company, is directed to a process for polymerizing monomers which involves contacting the polymerizing monomers which involves contacting the polymerization feedstock stream with an adsorbent material to remove non-polymerizable hydrocarbons separated from the feedstock stream. More specifically, Cottle discloses the clean-up of butadiene for anionic polymerization, although he also discloses that the process can be used to purify other olefinic, dienic, styrenic streams, and even hydrocarbon solvents. Cottle begins his process by partially purifying the feed by hydrogenation and fractional distillation, and then contacts the resultant stream with an adsorbent which may be activated carbon. Although isobutylene is a component of the stream used in the example, as indicated in the Table, there is no teaching or suggestion that poly-isobutylene is a contaminant in the stream, or that poly-isobutylene would be adsorbed by the activated carbon.
U.S. Pat. No. 2,765,914, Seyfried, assigned to Esso Research and Engineering Company, is directed to a method for removing free sulfur from liquefied hydrocarbon gas, and more specifically by removing the elemental sulfur by adsorption of the sulfur on adsorbent carbon. Seyfried discloses treating a C.sub.1 -C.sub.6, non-aromatic, liquefiable hydrocarbon with activated carbon to remove sulfur. Although C.sub.1 -C.sub.6, non-aromatic, liquefiable hydrocarbons are broadly disclosed and claimed, liquefied petroleum gas is disclosed as being most preferred for purposes of treatment in accordance with the invention disclosed by Seyfried. Seyfried does not teach or suggest the attraction of branched-chain hydrocarbons, or poly-isobutylene, using activated carbon.
U.S. Pat. No. 4,734,273, Haskell, assigned to Shell Oil Company, is directed to a process for selectively sorbing trace amounts of oxygen from low molecular weight olefins and inert gases by contacting with high surface area particulate coal-derived activated carbon having high ash and moisture contents. 1- and 2-butene and isobutene are disclosed as being suitable for this process; and the oxygen which is removed is originally present in concentrations up to about 10 ppm. There is no teaching or suggestion that the activated carbon adsorbent would be effective to remove branched-chain hydrocarbons or poly-isobutylene from raffinate streams.
In the past, therefore, the PIB contaminated streams normally have been discarded because of the associated processing difficulties. It would be desirable if objectionable amounts of PIB could be removed to produce a substantially PIB-free C.sub.4 stream which could be utilized in further processing.